What's the difference between port class A and port class B?

IO-Link masters could be two different port classes.

Port Class A (Type A)
This Port Class is specifically for sensors. There Pin 2 and Pin 5 is not specified. For this Pins the manufacturer defines the function. But Pin 2 is usually used with a digital channel.

Port Class B (Type B)
This Port Class is especially for actuators.
This Type provides additional supply voltage and is suitable for the connection of devices which have an increased current requirement. Here is on Pin2 and Pin 5 an additional supply voltage.